Working with ISO Metric Cylinders - Part 1

Sometime in the future, cylinders and other fluid power components will be built to ISO (International Standards Organization) dimensions in which metric measurements are used. Conversion to international standards has been slow in the United States, and at the time this sheet was prepared the availability of metric dimension cylinders was quite limited, but complete conversion will come in due time.

Cylinder force charts in this sheet cover standardized bore and rod combinations from 25mm through 200mm bore and with standard and maximum size piston rods. Intermediate size piston rods will no doubt be offered by most manufacturers. ISO standard sizes also include bore sizes of 8, 10, 12, 16, 20, 250, 320, and 400mm.

Calculations of cylinder force and velocity are not quite as straightforward as in the U. S. system because of extra conversions between units which become necessary. The ISO units which will be used in cylinder calculations are these:

FORCE. Force values are in Newtons (N). One Newton is equal to about 1/4 pound (0.2248 lb. to be exact), or 1 pound is equal to about 4½ N (4.448 N to be exact). This unit should serve for most cylinder calculations except where very large forces are involved in which the kilo Newton (kN) equal to 1,000 N may be used.

PISTON AREA. Piston bore is cataloged in units of millimeters (mm) as shown in the charts. For area, the mm² is too small for convenient calculations, so the unit for piston surface area will be the square centimeter (cm²). To calculate piston area, change bore diameter to em by dividing by 10. Then, find cm² piston area with the formula: A = πr² in the usual manner.

PRESSURE. Fluid pressure will usually be expressed in kilo Pascals (kPa) because the Pascal, which is defined as one Newton of force per square meter, is such a small unit that it is hard to work with in making calculations. One kPa = 1,000 Pa.

The bar is a more convenient unit for fluid pressure and will be allowed, at least for a limited time. The bar is related to the Pascal. One bar = 100,000 Pascals or 100 kPa. It is also equal to 14.5 PSI which is very close to one atmosphere. Pressure values in these charts are given in three pressure units, bars, kPa, and PSI, to help a person get a “feel” for the way metric pressure units compare with PSI units he has been using.

PUMP FLOW. Oil flow from a hydraulic pump is expressed in liters per minute (1/min). A liter is defined as one cubic decimeter (dm³), and is roughly ¼ gallon (0.2642 gallon to be exact). Or, 1 gal. = 3.785 liters. On very large flows, units of liters per second (lis) can be used.

FORCE CALCULATION. Cylinder force is calculated by multiplying piston surface area times fluid pressure:

F = A × P ÷ 70, in which:

F = force, in Newtons (N).
A = piston area in square centimeters (cm²).
P = differential pressure across ports in kPa.
10 is a necessary conversion between metric units.

When working with pressure in bars, the formula becomes:

F = A × P × 10, in which:

P is differential pressure, in bars.

METRIC HYDRAULIC CYLINDERS - FORCE CHART - 25 TO 175 BARS PRESSURE

METRIC HYDRAULIC CYLINDERS - FORCE CHART - 200 TO 350 BARS PRESSURE

METRIC AIR CYLINDERS - FORCE CHART - 3 TO 6 BARS PRESSURE

METRIC AIR CYLINDERS - FORCE CHART - 6½ TO 11 BARS PRESSURE

CROSSOVER - METRIC TO INCH BORE SIZES

© 1990 by Womack Machine Supply Co. This company assumes no liability for errors in data nor in safe and/or satisfactory operation of equipment designed from this information.